There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. CCDs are often employed for image acquisition in small size imaging applications. CCDs are also capable of large formats with small pixel size and they employ low noise charge domain processing techniques. However, CCD imagers have a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays.
Because of the inherent limitations in CCD technology, there is an interest in complementary metal oxide semiconductor (CMOS) imagers for possible use as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.
A typical CMOS imager includes a focal plane array of pixel cells, each one of the cells including either a photodiode, a photogate or a photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
There is a need for an effective light blocking structure in imagers which, as discussed below, can reduce the cross-talk and optical noise caused by stray light during image acquisition.
Accordingly, an improved imaging device capable of minimizing if not eliminating cross-talk and optical noise caused by stray light during image acquisition is needed. There is also a need for an improved method for fabricating imaging devices, in which there is reduced cross-talk and optical noise during image acquisition.